In control elements that can move back and forth between two switching positions, particularly fast-moving control elements, it is often necessary to determine the respective instantaneous speed and the respective instantaneous position of the switching element relative to the respective switching position in connection with a regulation and/or control during the movement of the control element. This is especially the case for determining the speed and position as the control element approaches the respective switching position.
Determining the speed and position is particularly significant in a control element that is connected to an armature that is moved back and forth, counter to the force of restoring springs, with the aid of an electromagnet or two spaced electromagnets. Because the force of the restoring springs that counteracts the movement increases linearly as the electromagnet approaches the respective switching position defined by the pole face of the electromagnet, the magnetic force increases progressively as the distance between the pole face and armature diminishes, and the armature moves with increasing speed toward the pole face with a constant current supply. As a result, the armature may not be held by the capturing electromagnet as it impacts the pole face, but may bounce backward. Depending on the magnitude of the impact speed, the armature can bounce completely away, so it is not held at all by the capturing electromagnet, or the armature bounces a short distance backward one or more times, but is always recaptured by the electromagnet. In both cases, however, it is no longer ensured that the system actuated by the control element will function properly. An example of a system embodied in this manner is a cylinder valve in a piston-type internal-combustion engine; the valve is connected to the armature, and can be opened and closed by way of two spaced electromagnets that are alternately supplied with current by a control device. A corresponding control of the current supply of the capturing electromagnet permits a reduction in the current at the capturing electromagnet as the armature approaches the pole face such that the respective effective magnetic force is slightly greater than the restoring force of the associated restoring spring, so the armature impacts the pole face xe2x80x9cgentlyxe2x80x9d with a speed of, for example, less than 0.1 m/sec.
To influence the control force acting on the control element such that the respective switching position is attained at a predeterminable speed, it is necessary to determine the speed curve using the control characteristic, on the one hand and, on the other hand, to determine the respective position of the control element, i.e., the control characteristic as a function of time, in order to influence the armature speed by changing the current supply. The use of two sensors, one of which determines the speed of the control element, while the other determines the course of the path as a function of time, offers an economical industrial application.
In the use of a path sensor, it is possible in principle to generate the speed signal from the path signal through differentiation. This is highly problematic, however, because the path signal includes noise components that cover the useful signal in the differentiation.
It is the object of the invention to provide a method that permits the determination of the speed and the path, as a function of timexe2x80x94referred to hereinafter as position xe2x80x94for a control element that can move back and forth, with only one sensor element.
In accordance with the invention, this object is accomplished by a method for determining the speed of movement and/or the position of a control element that can move back and forth between a first and a second switching position, in which an immersion or plunger body that is connected to the control element is guided synchronously with the movement of the control element by at least one stationary plunger coil, with the voltage that is generated due to a movement of the immersion body relative to a permanent magnet, and/or the movement of a permanent magnet relative to the plunger coil, being determined as a signal for the movement speed of the control element, and with changes in the impedance and/or a current that are caused by the movement of the immersion body in the plunger coil being determined as a signal for the position of the control element. This method capitalizes on the fact that, with a corresponding relative movement of the permanent magnet in the plunger coil, a voltage is induced that is a function of the rate of change of the magnetic flux, and thus of the movement speed. This voltage, or the course of the voltage, can be determined as a function of time, so a certain speed can be associated with each voltage value at a given time.
Because a separate immersion or plunger body is also moved synchronously with the movement of the control element in the plunger coil in addition to the permanent magnet, the effected changes in impedance, i.e., the changes in the impedance of the plunger coil, can be used to determine the respective position of the control element at any time. This is because the change depends on how far the immersion body dips into the plunger coil. The type of change depends on the electrical and/or magnetic properties of the immersion body. If the immersion body has a magnetic conductivity that is much greater than 1, the inductance changes, that is, a current flowing in the plunger coil changes its course as a function of time and the immersion depth. If the immersion or plunger body has a good electrical conductivity, the losses due to eddy-current effects in the plunger body change, which corresponds to an increase in the resistive component of the impedance. It is also possible, however, to determine the speed and position simultaneously using only one coil and the effect of the permanent magnet and the immersion body, without intermediate calculation steps.
Two possible procedures exist for determining the position. In a first embodiment of the invention, it is provided that the plunger coil is acted upon with a high-frequency alternating current, and the change in the current flow that is effected by eddy-current losses due to the movement of the immersion body in the plunger coil, the body at least partially comprising an electrically-conductive material, is determined as a signal for the position of the control element. The method capitalizes on the phenomenon of the generation of a magnetic alternating field when the coil is acted upon by an AC voltage, thereby effecting a current flow in the form of an eddy current in the electrically-conductive immersion body, which acts as a secondary winding. This current flow changes as a function of the immersion depth, that is, the position of the immersion body relative to the plunger coil. The properties of the coil, especially the AC behavior, also change, however, depending on the position of the immersion body. The position of the immersion body, and thus the position of the control element connected to the immersion body, can be ascertained through the determination of the impedance, or a value derived therefrom, such as the voltage, current or associated phase. The immersion body can be embodied as a separate element, or be formed by the control element itself. If the control element is produced from an electrically non-conductive material for some other reason, the immersion body comprising a conductive material is directly connected to the permanent magnet. In this instance, the electrically-conductive material of the immersion body should possess a relatively-low magnetic conductivity xcexc in order not to short-circuit the magnetic field of the permanent magnet. If the permanent magnet and the immersion body are disposed one behind the other in the direction of movement of the control element, the electrically-conductive material of the immersion body, as an xe2x80x9cextensionxe2x80x9d of the magnet in one direction, can even have a positive effect with respect to the linearity of the temporal course of the voltage in the coil as a function of the speed of movement of the permanent magnet.
The frequency of the eddy-current measurement is advantageously suited for the frequency range required for the voltage measurement, in other words, the movement frequency of the control element. If, for example, a maximum signal frequency of about 100 kHz results for the voltage measurement for determining the movement speed, it is advantageous when a frequency that is higher by at least a factor of 10, for example a frequency of about 1 MHz, is preset for the position detection by way of the ascertainment of the eddy-current losses. This ensures that the measurements have virtually no effect on one another.
In a further, advantageous embodiment of the invention, it is provided that a material that only effects perceptible eddy currents at higher AC-current frequencies is used for the immersion body. With this method step, the influence of the eddy currents on the speed signal induced by the permanent magnet is kept as small as possible. It is therefore possible that the changes in the induced voltages in the frequency range to be measured remain extensively unaffected in order to obtain not only the speed signal, but also a signal over the path course of the control element, that is, to determine the respective position of the control element, and to attain an improved dynamic precision. This is possible, for example, through a so-called bundling of the immersion body, or with an immersion body comprising a sintered material of a corresponding composition or corresponding structure, so perceptible eddy-current losses that can be used for signal generation occur in the range of the carrier frequencies, but not in the range of the measurement frequencies, in immersion bodies of this embodiment.
In another embodiment of the method of the invention, it is provided that the changes in inductance that are caused by the movement of the immersion body comprising a magnetically-conductive material in the plunger coil are detected as a signal for the armature position. It is also the case here that the inductance of the coil supplied with current likewise changes as a function of the position of the immersion body relative to the coil due to the change in the magnetic field. A corresponding signal can, again, be derived from this change.
In a further advantageous embodiment of the invention, it is provided that two plunger coils having different numbers of windings are used in the region of movement of the immersion body connected to the permanent magnet. This procedure is particularly advantageous when the change in the inductance is to be used to determine the position of the immersion body. It is useful here to preset a number of windings for the portion of the coil associated with the immersion body for determining the position of the control element that differs from the number of windings of the portion of the coil associated with the permanent magnet connected to the immersion body for determining the speed of movement. The two coils can be disposed on the same coil body. It is also possible, however, to provide a coil that has a so-called tap, i.e., one portion of the coil has a small number of windings and the other portion of the coil has a large number of windings. To keep the necessary cable lines small, one of the two coils can be bridged by a capacitor for the high frequencies, so two connections suffice for the coil system.
Further advantageous embodiments are disclosed and discussed in the ensuing description of embodiments.
The invention is described in detail below in conjunction with schematic drawings of embodiments.